The optical parametric conversion obtained in a non-linear optical medium enables for example “pump” photons, injected into the system at a frequency .ωp, to be converted into pairs of “signal” and “idler” photons at the different frequencies ωs and ωi. It enables twin photons to be generated, new frequencies to be generated (for the OPO) or weak signals to be amplified (for the OPA). Said effect is amplified when the non-linear medium is placed within a resonant cavity so as to produce an oscillator. It is known that when the cavity is close to the resonance for the “signal” and “idler” (doubly resonant OPO, or DROPO) or for the three waves (triply resonant OPO, or TROPO), the threshold for the parametric generation may be significantly reduced, which significantly increases the efficiency of the mechanism. It will be noted that one such system may generally operate under impulsional excitation. The OPO are sources of coherent light wherein the generation of new frequencies is spontaneous above a power threshold of the “pump”. The OPA, wherein the generation is not spontaneous, are used for the capacity thereof for amplifying the light beams, of low intensity, at the ωs or ωi frequency. The nature of the non-linear medium as well as the drawing of the cavities determines the ωs and ωi frequencies. For an active medium having an χ(2) type non-linear susceptibility, the conservation of energy results in the 2ωP=ωs+ωi relation. For an χ(3) type medium, the relation is 2ωp=ωs+ωi. In both cases, an effective parametric conversion requires optimisation of the phase agreement condition which is written as follows:Δk=kp−ki−ks=0 for an χ(2) type system or Δk=2kp−ki−k5=0 for an χ(3) type system.
Prior art knows monolithic or non-monolithic parametric conversion systems with the aid of cavities for obtaining a multi-resonance. The problem with the conventional non-monolithic OPO is the bulkiness thereof and complexity thereof because they require a pump laser, a non-linear crystal and external mirrors for the cavities. The difficulty for obtaining an effective parametric conversion is linked to the need to satisfy that which is known by the person skilled in the art as the phase agreement condition on one hand and on the other hand the necessity for obtaining resonant cavity modes with the parametric frequencies. Concerning the monolithic systems, two very different situations are differentiated depending on whether the non-linearity is in χ(2) or χ(3).
The publication by Savvidis, Baumberg, Stevenson, Skolnick, Whittaker and Roberts “Angle-Resonant Stimulated Polariton Amplifier”, published in Physical Review Letters, Vol 84, page 1547, February 2000, discloses a system for the parametric amplification via the means of a single planar microcavity. The electromagnetic field is entirely confined by the cavity according to the direction of growth and is moreover in strong exciton-photon coupling regime with the quantum well system placed in the core thereof. In said system, the χ(3) susceptibility is very high and enables an effective parametric conversion to be obtained for the ωp, ωs and ωi close frequencies, all included the stop band of the Bragg mirrors. The single cavity, in strong exciton-photon coupling regime, here enables a triple resonance to be obtained for the three frequencies. Nevertheless, the compliance of the phase agreement conditions requires that the injection of pump photons is carried out at a high value (typically)16° and delicate to determine particular angle, which makes any practical use difficult. In addition, the idler photons are emitted at an even higher angle, an angle for which the coupling of photons with the outside of the cavity is particularly low, which makes the generation of twin photons difficult. Finally, the strong exciton-photon coupling regime being essential for obtaining the phase agreement, the system only operates, with the materials available for the manufacturing, such as GaAs, at a low temperature.
On the other hand, for the systems using a non-linearity in χ(2), the publication by Haidar, Forget and Rosencher, “Optical Parametric Oscillation in Microcavities Based on Isotropic Semiconductors: a Theoretical Study”, published in IEEE Journal of Quantum Electronics, Vol 30, No 4, April 2003, discloses a system for the parametric oscillation wherein the reflectivity diagram of the mirrors used is such as illustrated in FIG. 1. Said document discloses a device wherein the cop, ws and wi frequencies are very different and may not all be included in the stop band of a single Bragg mirror. The resonance condition is therefore sought for the signal and the idler with the aid of two cavities not strongly coupled to one another in the direction defined further.
Patent application FR 2 751 796 is also known, in the name of the CEA, which describes a system of two coupled lasers wherein a first laser pumps a second laser. The laser cavities of said document are however not arranged for generating parametric frequencies. Indeed, the system described in D1 is not a parametric oscillator, but a simple laser. In particular, said document does not describe the nature of the coupling between the laser cavities and does not enable parametric frequencies to be managed.